The invention relates to increasing the lifetime of a laser gyro, and more particularly to improving the corrosion resistance of a seal ensuring the hermeticity of part of the structure of the laser gyro.
A laser gyro or gyrometer allows angular velocity to be measured. FIG. 1 illustrates a schematic representation of the structure of a laser gyro.
Typically, a laser gyro comprises a rigid structure called an optical bloc 1 that is polygonal shaped, often triangular or square. The optical bloc 1 comprises a glass-ceramic material with an elastic expansion coefficient that is almost zero. In other words, over the operating temperature range of the laser gyro i.e. between −60° C. and 100° C., the glass-ceramic first material does not expand and does not contract. Specifically, the glass-ceramic first materials comprise amorphous first materials that expand, crystalline second materials that contract and mobile ions that can move between the first and second materials. Expansion of the first materials offsets the contraction of the second materials of the glass-ceramic material.
The optical bloc 1 comprises a capillary tube 2 in which a laser beam travels. The capillary tube 2 is kept under an atmosphere comprising gases such as helium or neon. Mirrors 3 are placed at each corner of the optical bloc 1 so as to transmit the laser beam. The capillary tube 2 filled with a gas mixture and the mirrors form an amplification medium for the laser beam.
The optical block 1 contains apertures 4a and the capillary tube 2 contains apertures 4b, the apertures 4a and the apertures 4b being placed facing each other so as to allow electrodes 5a and 5b to pass, which electrodes penetrate into the capillary tube 2 and generate plasma discharges inside the capillary tube 2. The anode 5a and the cathode 5b comprise a metallic material generally comprising materials such as aluminium or Invar, which are materials that have a non-negligible elastic expansion coefficient.
The hermeticity of the capillary tube 2 is ensured by a seal 6 comprising indium. Indium is known for its good corrosion resistance properties and for its mechanical properties. Specifically, this material remains flexible and malleable down to cryogenic temperatures, typically about −150° C. The seal 6 is located inside the apertures 4a in the optical block 1, thereby preventing leakage of the inert gases to the exterior, which would irreversibly damage the laser gyro.
A laser gyro is a piece of angular measurement apparatus generally intended to be fitted on board an aircraft or a submarine, for example. It may therefore be subjected to very corrosive environments such as the marine environment or to large temperature variations during the takeoff and landing of aircraft.
When the laser gyro is not in operation, the indium present on the surface of the seal, and which makes contact with oxygen in the air or water, forms a passivation layer comprising indium oxide that makes it insensitive to corrosion. When the laser gyro is in operation, the potential difference applied to the electrodes 5a and 5b inhibits the passivation of the indium of the seal 6, thereby making it sensitive to corrosion.
Moreover, the temperature variations to which the laser gyro is subjected cause the materials of the electrodes 5a, 5b to expand or contract. But, the glass-ceramic from which the optical block 1 is made has a very small expansion coefficient, leading to differential expansion between the materials of the electrodes 5a, 5b and the glass-ceramic material. The indium-comprising seal 6 compensates for the differential expansion of the two materials.
As the laser gyro is used, the indium-comprising seal 6 becomes mechanically fatigued causing micro-cracks to appear into which oxygen molecules present in the air or water infiltrate. A passivation layer forms inside the seal 6 creating oxide holes in the volume of the seal 6. The holes then together form micro-cracks.
To limit the risk of deterioration due to corrosion of the seal 6, two solutions have been proposed in the literature.
A first solution is proposed in patent application US 2004/0040941. It consists in hermetically isolating the entire laser gyro from the corrosive environment by placing it inside a package kept under an atmosphere comprising an inert gas.
Implementation of this solution is still very difficult and expensive because production of a package that is perfectly hermetic to the oxygen in air and water and that allows connectors to pass has still not been completely mastered.
An improvement to this solution consists in adding getters. Getters are easily oxidized metallic materials. In contact with oxygen they are preferentially oxidized to form a white deposit.
A second solution is proposed in the prior art. It consists in protecting the seal 6 locally by placing a protective barrier 8 on the seal 6. This barrier 8 may be an adhesive, a lacquer or a mastic that prevents oxygen from passing.
This solution has two drawbacks: on the one hand, the materials used degrade under the effect, notably, of temperature variations or the effect of the electric field applied to the electrodes 5a, 5b; on the other hand, the adhesives, lacquers or other materials placed on the surface of the seal to protect it from corrosion are not perfectly hermetic to oxygen. But, a very small concentration of oxygen may result in substantial corrosion of the seal 6.